Evaluation of dwell time influence in Brinell, Vickers and Knoop hardness measurements of metallic materials

This work investigates the influence of test force duration, or dwell time, on Brinell, Vickers and Knoop hardness measurements of metallic materials. The experimental campaign includes 50 hardness configurations, combining different hardness scales, force levels, indentation geometries, materials and nominal hardness levels. Measurements are mainly performed at INRiM, with additional independent supporting datasets provided by PTB and TUBITAK UME. For each configuration, hardness values are evaluated at six dwell times, from 5 s to 50 s, giving a total of 300 indentations. The dependence of hardness on dwell time is modelled as a linear function of ln( t ). Sensitivity coefficients are estimated by weighted least-squares regression, using hardness uncertainties that include indentation measurement uncertainty and material inhomogeneity. The statistical significance level of each coefficient is assessed by a one-sided t -test. Results show that dwell time is a measurable influence factor in indentation hardness metrology, but its observability depends on the balance between the physical dwell-time effect and the dispersion caused by specimen inhomogeneity. Therefore, a higher p -value does not necessarily imply that dwell time is negligible. In several cases, physically consistent negative coefficients are obtained but with large uncertainties. The clearest trends are observed for macro-Vickers HV 30 and selected Brinell configurations, whereas low-force Vickers and Knoop measurements are more strongly affected by local heterogeneity and optical measurement uncertainty. The proposed approach supports the use of scale-specific sensitivity coefficients together with their uncertainties, rather than relying only on statistical significance, for dwell-time correction and uncertainty propagation in Brinell, Vickers and Knoop hardness metrology.